A manufacturing process for the production of absorbent products such as diapers, incontinence protectors, sanitary towels and panty liners normally comprises a processing of various continuous material webs, which are fed out from rolls or the like and passed through various work stations for the carrying out of various work elements and process steps. For example, it is common for a manufacturing process for absorbent products to provide a first material web that defines a backing layer consisting of a plastic film that is non-permeable to liquid and a second material web that defines an outer layer consisting of a liquid-permeable material, for example a non-woven material. The product can also be provided with other components, such as for example an absorbent core of a material intended to absorb bodily fluids.
The work elements that are carried out during a process of the type described above can consist, for example, of attaching together two or more layers of material, perforating, cutting, gluing, embossing a pattern or other type of shaping and processing of the materials involved. Other examples of work elements are the application of different components, such as fastening devices (so called tabs), cellulose centres, elastic material, so-called disposal tapes, etc.
In all, the material webs in question go through various process steps that result in a continuous complete material web that consists of a continuous row or strip of a number of absorbent products. Each individual product is finally shaped by this web being cut at regular intervals that correspond to the length of the finished product.
In a process of the type described above, some form of decorative element is often applied, such as a printed pattern or pictures, which are intended to enhance the visual impression of the finished product. Such a printing process is preferably carried out by conventional multi-colour printing. In particular, concerning absorbent products in the form of diapers for babies, such printed motifs, for example in the form of fairy-tale characters and cartoon characters, are considered to make the product more appealing to the consumer. In addition, such a procedure for printing a motif is suitably carried out on the backing layer for a diaper, not least due to the fact that such a backing layer is normally made of a polymer film that is essentially non-permeable to liquid, the surface of which is suitable for colour printing with a good quality and high resolution. In this way, a printed back is obtained on the finished product.
Certain types of printed motif are of such a nature that they can be positioned and oriented in any way on the back of the product. Such a printed motif can then be said to be “unsynchronized” in the sense that it does not need to be positioned in a given and precise way along the back of each product. This can, for example, be the case with an irregular pattern or a motif in the form of abstract symbols, the location of which on the back does not need to have a particular geometrical positioning on the product concerned.
There are, however, other types of printed motifs that can be said to be “synchronized” in the sense that they must be placed in a given position on the layer in question so that each individual product is provided with a print that is always in a predetermined position. An example of such a synchronized print can be a motif that is intended to be printed in the middle of the back of the product, that is centred both longitudinally and laterally.
Against the background of the above, it has been found that there is a need for simple, reliable and cost-effective methods and arrangements that have a high level of precision and using which a synchronized print in the form of patterns, characters and other motifs can be provided on an absorbent article. More specifically, the material web that carries the print in question is to be synchronized in an arrangement for manufacturing the product in question, so that the various work elements that are carried out on the product are carried out in the correct positions in relation to the printed motif.
A previously known way of obtaining such a synchronized printing process is to utilize previously printed reference marks or synchronizing marks that are suitably positioned at regular intervals on the material web in question. Each synchronizing mark can be printed as a small strip of colour along the edge of the material web and can be detected electronically by means of an optical detector. Such synchronizing marks are then used to control the manufacturing process for the product concerned so that, in its final position, the motif that is to appear on the finished product is always in the intended position on the finished product.
It can be noted that the known technology is based on a synchronizing mark having to be positioned well separated from the actual motif, so that the detecting device that is used to detect the synchronizing mark is not also triggered by the motif that is printed on the product in question. This limits the available area within which a printed motif can be applied, which is of course a disadvantage associated with the known technology.
Patent document WO 00/59429 shows an arrangement that utilizes synchronizing marks for controlling the positioning of a printed motif on an absorbent product. According to this document, synchronizing marks are provided on such sections of the product that are cut away later during the manufacturing process. In this way, temporary synchronizing marks are defined that are removed before the product has been completed.
In addition, document WO 99/32384 shows an arrangement for synchronizing two material webs during the manufacture of absorbent products. One of these material webs consists of a backing layer that comprises printed motifs, which are then to be synchronized with an additional material web that comprises an outer layer and an absorbent core. According to WO 99/32384, a stretching of the backing layer is obtained when required, with the object of synchronizing the two material webs.
Against the background of the abovementioned known technology, it can be pointed out that there is a need for methods and arrangements for improved synchronization of printed motifs and similar elements on absorbent products of various kinds, in particular for a precise detection of a synchronizing mark that is used in such a process for synchronization. For example, there is a desire to utilize an ever-increasing part of the material web concerned for printing various types of motif. This desire is in contrast to the requirement for a reliable detection of a synchronizing mark by positioning it a relatively short distance away from the printed motif on the absorbent product, as otherwise too large a motif could be interpreted as a synchronizing mark during the actual detection of the latter.